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Abstract

Cell polarity is a common feature of most eukaryotic cells. Recent studies revealed that two heterotrimeric protein complexes, Crumbs (Crb)- Stardust (Sdt)-Patj (Crb complex) and Par-6-aPKC-Bazooka (Baz, Par-3) (Par complex), play essential roles in the establishment and maintenance of apical-basal cell polarity. Using Drosophila photoreceptor as a model system, this dissertation identified three potential partners of these cell polarity proteins, which are Spectrins, Spastin and Centrosomin, and examined their roles in the photoreceptor morphogenesis.
Spectrins are major proteins in the cytoskeletal network. Although Spectrins are dispensable for retinal differentiation in eye imaginal discs during larval stage, photoreceptors deficient in α- and β-Spectrins display dramatic apical membrane expansions during pupal eye development. βHeavy-Spectrin (Karst) localizes apically, while β-Spectrin is preferentially distributed in the basolateral region. Overexpression of β-Spectrin causes a strong shrinkage of apical membrane domains, and loss of β-Spectrin causes an expansion of apical domains, implying an
antagonistic relationship between β-Spectrin and Karst. These results indicate that Spectrins are required for controlling photoreceptor morphogenesis through the modulations of cell membrane domains.
By using of anti-acetylated-tubulin antibody, I found that there are stable microtubules in the developing photoreceptors of Drosophila. Spastin is a microtubule-severing ATPase involved in constructing neuronal and non-centrosomal microtubule arrays. The spastin mutation causes mild defects at the distal section, but the apical domain was dramatically reduced at the proximal section of the developing pupal eye. Spastin overexpression caused the expansion of the apical membrane domain from apical to basolateral in the developing photoreceptor. These results strongly suggest that spastin is essential for apical domain biogenesis during rhabdomere elongation in Drosophila photoreceptor morphogenesis.
Centrosomin (Cnn) is a core protein for centrosome. I found that Cnn is dispensable for retinal differentiation in eye imaginal discs during the larval stage. However, photoreceptors deficient in Cnn display dramatic morphogenesis defects including the mislocalization of Crb and Baz during pupal eye development. Cnn overexpression caused the expansion of the apical Crb membrane domain, Baz and adherens junctions. These results strongly suggest that the interaction of Baz and Cnn is essential for apical domain and adherens junctions modulation during photoreceptor morphogenesis.